Glucosidase enzymes are involved in several biological processes such as the intestinal digestion, the biosynthesis of glycoproteins and the lysosomal catabolism of the glycoconjugates (Homonojirimycin isomers and N-alkylated homonojirimycins: structural and conformational basis of inhibition of glycosidases. Asano N, Nishida M, Kato A, Matsui K, Shimada Y, Itoh T, Baba M, Watson A A, Nash R J, Lilley P M, Watkin D J, Fleet G W, J Med Chem. 1998 Jul. 2; 41(14):2565-71). Intestinal α-glucosidases are involved in the final step of the carbohydrate digestion to convert these into monosaccharides which are absorbed from the intestine.
As a result of the catalysis produced by α-glucosidase enzyme in the final step in the digestive process of carbohydrates, its inhibitors can retard the uptake of dietary carbohydrates and suppress postprandial hyperglycemia, and could be useful to treat diabetic and/or obese patients [Novel α-glucosidase Inhibitors with a tetrachlorophthalimide Skeleton., S. Sou, S. Mayumi, H. Takahashi, R. Yamasak, S. Kadoya, M. Sodeoka, and Y. Hashimoto, Bioorg. Med. Chem. Lett., 2000, 10, 1081].
The α-glucosidase inhibitors are effective in lowering the insulin release, insulin requirement and some can lower plasma lipids. The acarbose is a very widely prescribed drug in the management of the type II diabetes and recently a U.S. Pat. No. 6,387,361 to Rosner describes the use of acarbose in the treatment of obesity. According to the criteria issued by WHO (World Health Organization) based on a glucose tolerance test, diabetes mellitus and impaired glucose tolerance (hereinafter sometimes referred to as IGT) are distinguished by the fasting blood glucose level and the blood glucose level 2 hours after glucose loading. Patients with IGT have high blood glucose levels compared to those of patients with diabetes mellitus, and are reported to be at increased risk of developing diabetes mellitus and complications of arteriosclerotic diseases. In particular, it is known that patients with IGT who have blood glucose levels of 170 mg/dl or above at 2 hours following glucose loading, i.e., patients with high-risk IGT, may develop diabetes mellitus at a high rate [Diabetes Frontier, p. 136, 1992]. With regard to voglibose which is an α-glucosidase inhibitor, there are reports of studies on effects of voglibose for insulin-resistant IGT and diabetes [Yakuri-to-Chiryo (Japanese Pharmacology & Therapeutics), 24 (5):213 (1996); Metabol. Exp. Clin., 45:731, 1996]. Voglibose (AO-128) is also known to have effects of lowering blood glucose level and improving glucose tolerance in rats [Yakuri-to-Chiryo (Japanese Pharmacology & Therapeutics), 19 (11):161 (1991); Journal of Nutrition Science and Vitaminology, 45 (1):33 (1992)]. On the contrary, it has also been reported that the effect of voglibose in improving glucose tolerance could not be verified in human [Rinsho-Seijinbyo, 22 (4): 109 (1992)]. An antibiotic pradimicin Q as α-glucosidase inhibitor is described in the U.S. Pat. No. 5,091,418 to Swada.
In addition, they have also been used as antiobesity drugs, fungistatic compounds, insect antifeedents, antivirals and immune modulators [Glycosidase inhibitors and their chemotherapeutic value, Part 1. el Ashry E S, Rashed N, Shobier A H., Pharmazie. 2000 April; 55(4):251-620]. The antiviral activity due to inhibition of α-glucosidase results from abnormal functionality of glycoproteins because of incomplete modification of glycans. Suppression of this process is the basis of antiviral activity [A glucosidase-Inhibitors as potential broad based antiviral agents, Anand Mehta, Nicole Zitzmann, Pauline M. Rudd, Timothy M. Block, Raymond A. Dwek, Febs Letters 430 (1998)17-22] and decrease in growth rate of tumors [Inhibition of experimental metastasis by an alpha-glucosidase inhibitor, 1,6-epi-cyclophellitol. Atsumi S, Nosaka C, Ochi Y, Iinuma H, Umezawa K. Cancer Res. 1993 Oct. 15; 53(20):4896-9]. The α-glucosidase inhibitor N-(1,3-dihydroxy-2-propyl)valiolamine is described as a promoter of calcium absorption in the U.S. Pat. No. 5,036,081.
Lichens are small perennial plants consisting of a symbiotic association of a fungus and an alga. They produce characteristic secondary metabolites that are unique with respect to those of higher plants. Several lichen extracts have been used for various remedies in folk medicine, and screening test with lichens have indicated the frequent occurrence of metabolites with antibiotic (Cavallito, C. J.; Fruehauf, D. M.; Bailey, J. H. J. Am. Chem. Soc. 1948, 70, 3724-3726), anti-mycobacterial, (Ingolfsdottir, K.; Chung, G. A. C.; Skulason, V. G.; Gissurarson, S. R.; Vilhelmsdottir, M. Eur. J. Pharm. Sci. 1998, 6, 141-144), antiviral (Yamamoto, Y.; Miura, Y.; Kinoshita, Y.; Higuchi, M.; Yamada, Y.; Murakami, A.; Ohigashi, H.; Koshimizu, K. Chem. Pharm. Bull. 1995, 43, 1388-1390; Neamati, N.; Hong, H.; Mazumder, A.; Wang, S.; Sunder, S.; Nicklaus, M. C.; Milne, G. W.; Proksa, B.; Pommier, Y. J. Med. Chem. 1997, 40, 942-951), analgesic and antipyretic properties (Okuyama, E.; Umeyama, K.; Yamazaki, M.; Kinoshita, Y.; Yamamoto, Y. Planta Med. 1995, 61, 113-115). Other literature reports of biological activities of these naturally occurring compounds are scarce, thus the therapeutic potential of lichens remain largely unexplored. This may partly be due to the difficulties encountered with collecting substantial amounts of plant material, as most of the lichen species grow as scattered patches, mainly on stones or on tree trunks. The study of bioactivities of lichen compounds is important because the secondary metabolites of lichens are found almost exclusively only in lichens. Out of the ≈700 secondary metabolites know up to 80% are restricted to the lichenized state. (Huneck, S. and Yoshimura, I. 1996, Identification of Lichen substances. Springer). Furthermore these distinct classes of lichen metabolites have not been fully tested for their enzyme inhibitory assays.
Herein we report the surprising α-glucosidase inhibition activity of three lichen metabolites that can be used as potent α-glucosidase inhibitor and thus in a therapeutic modality in a variety of medical applications as described above. These include Methylorsellinate or 2,4-dihydroxy-6-methylbenzoate (Compound I) isolated from Parmotrema grayana, and Roccella montagnei reported from Pseudocyphellaria crocata (L.) Vain (Maass, W. S. G.; Can. J. Bot., 1975b, 53, 1031-1039) Methyl-β-orinolcarboxylate or 2,4-dihydroxy-3,6-dimethylbenzoate (Compound II) isolated from Cladonia sp. reported from Stereocaulan alpinum Laur., (Hylands, P. J.; Ingolfsdottir, K.; Phytochemistry, 1985, 24, 127-129) and Zeorin or 6,22-Hopanediol (Compound III) (a constituent of various lichens e.g. Anaptychia, Lecanora, Parmelia, Nephroma, Placodium sp.) isolated from Cladonia sp. Other than the report on isolation of zeorin from Iris missouriensis roots (Wong, S. M.; Oshima, Y.; Pezzuto, J. M.; Fong, H. H.; Fransworth, N. R.; J. Phar. Sci, 1986, 75(3), 317-320) these compounds are lichen specific.
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